EP 2 541 572 A1 describes, on the one hand, an on-load tap changer for connection with a control winding of a transformer. This on-load tap changer comprises a selector and a load changeover switch connected with the selector. The selector comprises a plurality of fixed contacts connected with associated taps of the control winding, two moved contacts and a current collector for each moved contact. Each moved contact is connected by one end with the respective current collector and can be moved longitudinally of the current collector to different positions in which it is electrically connected at its other end with an associated fixed contact. The load changeover switch has two branches each having a series circuit of a main isolating switch and a transition isolating switch as well as a transition resistance connected in parallel with the main isolating switch. Due to this series circuit, this load changeover switch is also termed series load changeover switch. Each branch is connected between an outer contact and an associated current collector. The main isolating switch is switched in such a way that it lies electrically closer to the outer contact than the transition isolating switch. The transition resistances are connected with the outer contact through a common line. The on-load tap changer additionally comprises at least one current detector and fault detector. Each current detector is adapted to detect a current flowing through a path of the load changeover switch and, in response to the detected current, generate a current indication signal. The fault detector is adapted to receive the current indication signal from the current detector, to determine on the basis of the received current indication signal whether a fault has occurred in the load changeover switch and to generate a fault indication signal if it is determined that a fault has occurred in the load changeover switch. The fault detector is adapted to determine whether a transition isolating switch has failed to open in that it determines whether the duration of the current indication signal exceeds a first threshold value. The fault detector is adapted to determine whether a main isolating switch has failed to open in that it determines whether the duration of the current indication signal is shorter than a second threshold value. Each current detector is a current measuring device that with the help of the current indication signal indicates the strength of a detected current and the fault detector is adapted to take into consideration the strength of the detected current in the determination of whether a fault has occurred. The current detector is adapted to detect the current flowing through the common line. The load changeover switch has for each branch a current detector adapted to detect the current flowing through the transition resistance of this branch and for each branch a current detector adapted to detect the current flowing through the transition isolating switch of this branch. The fault detector is adapted to determine whether a transition isolating switch has failed to open in that it checks whether the time period during which a current detection signal is received from the current detector from two branches exceeds a third threshold value. The fault detector is connected with a trigger mechanism of a transformer protective switch so that the transformer, with which the on-load tap changer is connected, can be separated from the mains if a serious fault, typically a fault in a transition isolating switch, should be detected.
EP 2 541 572, on the other hand, describes a method of operating an on-load tap changer with a load changeover switch that has two branches. Each branch comprises a series circuit of a main isolating switch and a transition isolating switch, and a transition resistance connected in parallel with the main isolating switch. This method comprises the steps that                at least one path of the load changeover switch is monitored in order to detect current flowing through the path;        a current indication signal is generated in response to the current flowing through the path;        based on the generated current indication signal it is determined whether a fault has occurred in the load changeover switch; and        a fault indication signal is generated if it was determined that a fault had occurred in the load changeover switch.        
The monitoring embraces at least one path monitored with regard to whether a current flows through a transition resistance. The determination comprises determining whether the duration of a received current indication signal exceeds a first threshold value and is shorter than a second threshold value.
The method comprises the step of determining the strength of a detected current. The determination of whether a fault has occurred is carried out in dependence on the determined magnitude of the detected current. The monitoring of at least one path comprises monitoring for each branch whether a current flows through transition isolating switch of that branch. The determination whether a fault has occurred comprises checking whether a time period during which a current indication signal from the current detector is received from two branches exceeds a third threshold value. The threshold values are predetermined.
EP 2 541 572 A1 describes that the on-load tap changer for changing the active winding number of the transformer switches over from a first continuous current state to a second continuous current state. In the first continuous current state the current flows from the first tap through the first moved contact, the first current collector and the first transition isolating switch and first main isolating switch of the first branch to the outer contact and a second tap is separated from the outer contact. In the second continuous current state the current flows from a second tap through the second moved contact, the second current collector and the second transition isolating switch and second main isolating switch of the second branch to the outer contact and the first tap is separated from the outer contact. Starting from the first continuous current state the first main isolating switch is opened so that the current now flows through the first transition resistance of the first branch as well as the first transition isolating switch. Thereafter, the second transition isolating switch is closed so that the load changeover switch now has two completed branches for the current flow, in which case the flow in each branch is limited by a transition resistance. Thereafter, the first transition isolating switch is opened so that now the entire current flows through the second transition resistance and the second transition isolating switch and still remains limited by the second transition resistance. Finally, the second main isolating switch is closed so that now the current flows through the second main isolating switch and the second transition isolating switch and the second continuous current state is reached.
Thus, each series circuit of transition isolating switch and main isolating switch forms a main path and each series circuit of transition isolating switch and transition resistance, which is connected in parallel with the main isolating switch, forms a transition path.
DE 20 21 575 describes a load changeover switch with two branches each having a parallel circuit of a first vacuum interrupter and a series circuit of a transition resistance and a second vacuum interrupter. Due to this parallel circuit this load changeover switch is also termed parallel load changeover switch. In each branch the first vacuum interrupter forms a main path and the series circuit forms a transition path.
DE 42 31 353 describes a load changeover switch with two vacuum interrupters, a transition resistance and a changeover switch that comprises two changeover contacts and a middle contact. The first vacuum interrupter is connected between middle contact and load output line. The first changeover contact is connected with a first moved contact of a selector and the second changeover contact is connected with a second moved contact. A series circuit of transition resistance and second vacuum interrupter is connected between second moved contact and load output line and thus parallel to the first vacuum interrupter, middle contact and second changeover contact. The first vacuum interrupter, the middle contact and the first changeover contact form a first main path, the series circuit forms a transition path, and the first vacuum interrupter, the middle contact and the second changeover contact form a second main path.
DE 42 31 353 A1 additionally describes a load changeover switch with two branches, of which one comprises a first vacuum interrupter and the other a parallel circuit of a second vacuum interrupter and a series circuit of a transition resistance and a third vacuum interrupter. The first vacuum interrupter forms a first main path, the series circuit forms a transition path and the second vacuum interrupter forms a second main path.
DE 10 2007 004 530 A1 describes a load changeover switch with three vacuum interrupters, two transition resistances and a changeover switch that comprises two changeover contacts and a middle contact. The first vacuum interrupter is connected between middle contact and load output line. The first changeover contact is connected with a first moved contact of a selector and the second changeover contact is connected with a second moved contact. A first series circuit of first transition resistance and second vacuum interrupter is connected between first moved contact and load output line and thus parallel to first vacuum interrupter, middle contact and first changeover contact. A second series circuit of second transition resistance and third vacuum interrupter is connected between second moved contact and load output line and thus parallel to first vacuum interrupter, middle contact and second changeover contact. The first vacuum interrupter, the middle contact and the first changeover contact form a first main path, the first series circuit forms a first transition path, the second series circuit forms a second transition path and the first vacuum interrupter, the middle contact and the second changeover contact form a second main path.